Internal Workings 143 



Behind the mouth is the pharynx. Here there is a con- 

 striction open only when food is passing through into the 

 stomach. If it were not for this barrier, the fish's stomach 

 would be continually flooded with water, and the digestive 

 juices would be diluted and rendered useless. The stomach 

 is generally U-shaped or in the form of a sack, and is highly 

 distensible. The intestine in the carnivores is comparatively 

 short, being hardly more than a straight tube leading to the 

 anus, but in the herbivores, whose food is in a form not so 

 readily assimilated, it may be long and twisted. In the carp 

 it is eight to nine times the overall length of th^ fish. The 

 majority of herbivores are fresh-water inhabitants. 



Close to the stomach are blind pouches, like the fingers of 

 a glove, known as caeca. They are found in no vertebrates 

 above the fish, although they do superficially resemble our 

 vermiform appendix. However, they attach to a much more 

 forward portion of the intestine than our expensive organ 

 does. The catfish and the pikes are without caeca. The flat- 

 fishes have 2 or 3. Our childhood friend the sunfish has 7 

 to 10, and his relatives the black basses have about 14. The 

 brook trout has 40 to 50, the Atlantic salmon 50 to 60, the 

 steelhead 42 to 80. The Pacific salmons vary from as low as 

 45 in the silver to as high as 214 in the king. But the tuna 

 and the swordfish head the listj in these species the caeca are 

 described as "very numerous," which we may take as mean- 

 ing that there are too many for a busy ichthyologist to bother 

 counting. 



The caeca probably serve to retain the food for digestive 

 and absorptive purposes. Similar in action is the geometrically 

 admirable spiral valve (see Figure 20), especially charac- 

 teristic of the sharks and rays. It lies at the lower end of the 

 gut. It works just as a long, twisted intestine would in pro- 

 viding more absorptive surface and greater storage capacity. 



